Using In Situ Optical Spectroscopy to Elucidate Film Formation of Metal Halide Perovskites.

The research interest in halide perovskites has gained momentum enormously over the last recent years, also due to the demonstration of high-efficient perovskite-based optoelectronic devices. A prerequisite for such highly efficient devices is to realize high-quality perovskite layers, which requires a deep understanding about the perovskite formation and good process control. In that context, in situ optical spectroscopy during the processing of halide perovskites has become increasingly popular. Even though it is a relatively easily accessible yet powerful tool for studying perovskite formation, there exist some technical and analytical aspects that need to be considered to unfold its full potential. In this Perspective, we give an overview of the latest developments in the field of in situ optical spectroscopy to control and better understand the film processing of halide perovskites. We highlight possibilities and pitfalls regarding the analysis of measured optical data, discuss the development of technical concepts, and address future prospects of optical in situ spectroscopy.

Disorder in P3HT Nanoparticles Probed by Optical Spectroscopy on P3HT-b-PEG Micelles.

We employ photoluminescence (PL) spectroscopy on individual nanoscale aggregates of the conjugated polymer poly(3-hexylthiophene), P3HT, at room temperature (RT) and at low temperature (LT) (1.5 K), to unravel different levels of structural and electronic disorder within P3HT nanoparticles. The aggregates are prepared by self-assembly of the block copolymer P3HT-block-poly(ethylene glycol) (P3HT-b-PEG) into micelles, with the P3HT aggregates constituting the micelles' core. Irrespective of temperature, we find from the intensity ratio between the 0-1 and 0-0 peaks in the PL spectra that the P3HT aggregates are of H-type nature, as expected from π-stacked conjugated thiophene backbones. Moreover, the distributions of the PL peak ratios demonstrate a large variation of disorder between micelles (inter-aggregate disorder) and within individual aggregates (intra-aggregate disorder). Upon cooling from RT to LT, the PL spectra red-shift by 550 cm-1, and the energy of the (effective) carbon-bond stretch mode is reduced by 100 cm-1. These spectral changes indicate that the P3HT backbone in the P3HT-b-PEG copolymer does not fully planarize before aggregation at RT and that upon cooling, partial planarization occurs. This intra-chain torsional disorder is ultimately responsible for the intra- and inter-aggregate disorder. These findings are supported by temperature-dependent absorption spectra on thin P3HT films. The interplay between intra-chain, intra-aggregate, and inter-aggregate disorder is key for the bulk photophysical properties of nanoparticles based on conjugated polymers, for example, in hierarchical (super-) structures. Ultimately, these properties determine the usefulness of such structures in hybrid organic-inorganic materials, for example, in (bio-)sensing and optoelectronics applications.

Direct observation of backbone planarization via side-chain alignment in single bulky-substituted polythiophenes.

The backbone conformation of conjugated polymers affects, to a large extent, their optical and electronic properties. The usually flexible substituents provide solubility and influence the packing behavior of conjugated polymers in films or in bad solvents. However, the role of the side chains in determining and potentially controlling the backbone conformation, and thus the optical and electronic properties on the single polymer level, is currently under debate. Here, we investigate directly the impact of the side chains by studying the bulky-substituted poly(3-(2,5-dioctylphenyl)thiophene) (PDOPT) and the common poly(3-hexylthiophene) (P3HT), both with a defined molecular weight and high regioregularity, using low-temperature single-chain photoluminescence (PL) spectroscopy and quantum-classical simulations. Surprisingly, the optical transition energy of PDOPT is significantly (∼2,000 cm-1 or 0.25 eV) red-shifted relative to P3HT despite a higher static and dynamic disorder in the former. We ascribe this red shift to a side-chain induced backbone planarization in PDOPT, supported by temperature-dependent ensemble PL spectroscopy. Our atomistic simulations reveal that the bulkier 2,5-dioctylphenyl side chains of PDOPT adopt a clear secondary helical structural motif and thus protect conjugation, i.e., enforce backbone planarity, whereas, for P3HT, this is not the case. These different degrees of planarity in both thiophenes do not result in different conjugation lengths, which we found to be similar. It is rather the stronger electronic coupling between the repeating units in the more planar PDOPT which gives rise to the observed spectral red shift as well as to a reduced calculated electron-hole polarization.

Structural Diversity in Layered Hybrid Perovskites, A2PbBr4 or AA'PbBr4, Templated by Small Disc-Shaped Amines.

We present three new hybrid layered lead(II) bromide perovskites of generic composition A2PbBr4 or AA'PbBr4 that exhibit three distinct structure types. [TzH]2PbBr4 ([TzH+] = 1,2,4-triazolium) adopts a (001)-oriented layer structure and [AaH]2PbBr4, ([AaH+] = acetamidinium) adopts a (110)-oriented type, whereas [ImH][TzH]PbBr4, ([ImH+] = imidazolium) adopts a rare (110)-oriented structure with enhanced corrugation (i.e., 3 × 3 type). The crystal structures of each are discussed in terms of the differing nature of the templating molecular species. Photoluminescent spectra for each are reported and the behaviors discussed in relation to the different structure of each composition.

Spin-Crossover Iron(II) Coordination Polymer with Fluorescent Properties: Correlation between Emission Properties and Spin State.

A spin-crossover coordination polymer [Fe(L1)(bipy)]n (where L = a N2O22- coordinating Schiff base-like ligand bearing a phenazine fluorophore and bipy = 4,4'-bipyridine) was synthesized and exhibits a 48 K wide thermal hysteresis above room temperature (T1/2↑ = 371 K and T1/2↓ = 323 K) that is stable for several cycles. The spin transition was characterized using magnetic measurements, Mössbauer spectroscopy, and DSC measurements. T-dependent X-ray powder diffraction reveals a structural phase transition coupled with the spin transition phenomenon. The dimeric excerpt {(µ-bipy)[FeL1(MeOH)]2}·2MeOH of the coordination polymer chain crystallizes in the triclinic space group P1̅ and reveals that the packing of the molecules in the crystal is dominated by hydrogen bonds. Investigation of the emission properties of the complexes with regard to temperature shows that the spin crossover can be tracked by monitoring the emission spectra, since the emission color changes from greenish to a yellow color upon the low spin-to-high spin transition.

Setup to Study the in Situ Evolution of Both Photoluminescence and Absorption during the Processing of Organic or Hybrid Semiconductors.

In situ measurement techniques, applied during the solution processing of novel semiconductors such as organic semiconductors or hybrid perovskites, have become more and more important to understand their film formation. In that context, it is crucial to determine how the optical properties, namely photoluminescence (PL) and absorption, evolve during processing. However, until now PL and absorption have mostly been investigated independently, significantly reducing the potential insights into film formation dynamics. To tackle this issue we present the development of a detection system that allows simultaneous measurement of full absorption and PL spectra during solution processing of the investigated film. We also present a spin-coater system attachable to the detection system, where the temperature of the substrate on which the film is processed can be changed. We performed test measurements by spin coating the well-known conjugated polymer P3HT demonstrating the potential of this technique. By considering absorption and corresponding PL, we extract the PL quantum yield (PLQY) during processing, which decreases with substrate temperature. Furthermore, we identify a significant red shift of the PL just prior to the onset of the aggregation process, indicating the importance of chain planarization prior to solid film formation.

The role of PbI2 in CH3NH3PbI3 perovskite stability, solar cell parameters and device degradation.

We report a systematic investigation on the role of excess PbI2 content in CH3NH3PbI3 perovskite film properties, solar cell parameters and device storage stability. We used the CH3NH3I vapor assisted method for the preparation of PbI2-free CH3NH3PbI3 films under a N2 atmosphere. These pristine CH3NH3PbI3 films were annealed at 165 °C for different time intervals in a N2 atmosphere to generate additional PbI2 in these films. From XRD measurements, the excess of PbI2 was quantified. Detailed characterization using scanning electron microscopy, X-ray diffraction, UV-Visible and photoluminescence for continuous aging of CH3NH3PbI3 films under ambient condition (50% humidity) is carried out for understanding the influence of different PbI2 contents on degradation of the CH3NH3PbI3 films. We find that the rate of degradation of CH3NH3PbI3 is accelerated due to the amount of PbI2 present in the film. A comparison of solar cell parameters of devices prepared using CH3NH3PbI3 samples having different PbI2 contents reveals a strong influence on the current density-voltage hysteresis as well as storage stability. We demonstrate that CH3NH3PbI3 devices do not require any residual PbI2 for a high performance. Moreover, a small amount of excess PbI2, which improves the initial performance of the devices slightly, has undesirable effects on the CH3NH3PbI3 film stability as well as on device hysteresis and stability.

The effect of intermolecular interaction on excited states in p-DTS(FBTTH2)2.

Using optical spectroscopy in solution and thin film, and supported by quantum chemical calculations, we investigated the aggregation process of the donor-acceptor type molecule p-DTS(FBTTH2)2. We demonstrate that cooling a solution induces a disorder-order phase transition that proceeds in three stages analogous to the steps observed in semi-rigid conjugated polymers. By analyzing the spectra, we are able to identify the spectral signature of monomer and aggregate in absorption and emission. From this we find that in films, the fraction of aggregates is near 100% which is in contrast to films made from semi-rigid conjugated polymers.

Relaxation dynamics and exciton energy transfer in the low-temperature phase of MEH-PPV.

Understanding the effects of aggregation on exciton relaxation and energy transfer is relevant to control photoinduced function in organic electronics and photovoltaics. Here, we explore the photoinduced dynamics in the low-temperature aggregated phase of a conjugated polymer by transient absorption and coherent electronic two-dimensional (2D) spectroscopy. Coherent 2D spectroscopy allows observing couplings among photoexcited states and discriminating band shifts from homogeneous broadening, additionally accessing the ultrafast dynamics at various excitation energies simultaneously with high spectral resolution. By combining the results of the two techniques, we differentiate between an initial exciton relaxation, which is not characterized by significant exciton mobility, and energy transport between different chromophores in the aggregate.

A spectroscopic assessment of static and dynamic disorder in a film of a polythiophene with a planarized backbone.

The optoelectronic performance of organic semiconductor devices is related to the static and dynamic disorder in the film. The disorder can be assessed by considering the linewidth of its optical spectra. We focus on identifying the effect of conjugation length distribution on the static energetic disorder. Hence, we disentangle the contributions of static and dynamic disorder to the absorption and emission spectra of poly(3-(2,5-dioctylphenyl)-thiophene) (PDOPT) by exploring how the linewidth and energy of the spectra evolve upon cooling the sample from 300 K to 5 K. PDOPT has sterically hindered side chains that arrange such as to cause a planarized polymer backbone. This makes it a suitable model for a quasi-one-dimensional molecular system. By modelling the conjugated segments as coupled oscillators we find that the linewidth contribution resulting from the variation of conjugation length decreases linearly with decreasing exciton energy and extrapolates to zero at the energy corresponding to an infinite chain. These results provide a new avenue to the design of low disorder and hence high mobility polymeric semiconductors.

Disorder to order: how halide mixing in MAPbI3-xBrx perovskites restricts MA dynamics.

Mixed-halide lead perovskites are of particular interest for the design of tandem solar cells currently reaching record efficiencies. While halide phase segregation upon illumination of mixed perovskites is extensively studied, the effect of halide disorder on A cation dynamics is not well understood, despite its importance for charge carrier diffusion and lifetime. Here, we study the methylammonium (MA) reorientational dynamics in mixed halide MAPbI3-xBrx perovskites by a combined approach of experimental solid-state NMR spectroscopy and molecular dynamics (MD) simulations based on machine-learning force-fields (MLFF). 207Pb NMR spectra indicate the halides are randomly distributed over their lattice positions, whereas PXRD measurements show that all mixed MAPbI3-xBrx samples are cubic. The experimental 14N spectra and 1H double-quantum (DQ) NMR data reveal anisotropic MA reorientations depending on the halide composition and thus associated disorder in the inorganic sublattice. MD calculations allow us to correlate these experimental results to restrictions of MA dynamics due to preferred MA orientations in their local Pb8I12-nBrn "cages". Based on the experimental and simulated results, we develop a phenomenological model that correlates the 1H dipolar coupling and thus the MA dynamics with the local composition and reproduces the experimental data over the whole composition range. We show that the dominant interaction between the MA cations and the Pb-X lattice that influences the cation dynamics is the local electrostatic potential being inhomogeneous in mixed halide systems. As such, we generate a fundamental understanding of the predominant interaction between the MA cations and the inorganic sublattice, as well as MA dynamics in asymmetric halide coordinations.

The Impact of Solvent Vapor on the Film Morphology and Crystallization Kinetics of Lead Halide Perovskites during Annealing.

One of the key factors for the remarkable improvements of halide perovskite solar cells over the last few years is the increased control over perovskite crystallinity and its thin film morphology. Among various processing methods, solvent vapor-assisted annealing (SVAA) has proven to be promising in achieving high-quality perovskite films. However, a comprehensive understanding of the perovskite crystallization process during SVAA is still lacking. In this work, we use a home-built setup to precisely control the SVAA conditions to investigate in detail the perovskite crystallization kinetics. By changing the solvent vapor concentration during annealing, the perovskite grain size can be tuned from 200 nm to several micrometers. We monitor the crystallization kinetics during solvent-free annealing and SVAA using in situ grazing incidence wide-angle X-ray scattering, where we find a diminished perovskite growth rate and the formation of low dimensional perovskite at the top of the perovskite layer during SVAA. Scanning electron microscopy images of the final films further suggest that the perovskite growth follows an Ostwald ripening process at higher solvent concentrations. Thus, our results will contribute to achieve a more targeted processing of perovskite films.

Microscopic (Dis)order and Dynamics of Cations in Mixed FA/MA Lead Halide Perovskites.

Recent developments in the field of high efficiency perovskite solar cells are based on stabilization of the perovskite crystal structure of FAPbI3 while preserving its excellent optoelectronic properties. Compositional engineering of, for example, MA or Br mixed into FAPbI3 results in the desired effects, but detailed knowledge of local structural features, such as local (dis)order or cation interactions of formamidinium (FA) and methylammonium (MA), is still limited. This knowledge is, however, crucial for their further development. Here, we shed light on the microscopic distribution of MA and FA in mixed perovskites MA1-x FA x PbI3 and MA0.15FA0.85PbI2.55Br0.45 by combining high-resolution double-quantum 1H solid-state nuclear magnetic resonance (NMR) spectroscopy with state-of-the-art near-first-principles accuracy molecular dynamics (MD) simulations using machine-learning force-fields (MLFFs). We show that on a small local scale, partial MA and FA clustering takes place over the whole MA/FA compositional range. A reasonable driving force for the clustering might be an increase of the dynamical freedom of FA cations in FA-rich regions. While MA0.15FA0.85PbI2.55Br0.45 displays similar MA and FA ordering as the MA1-x FA x PbI3 systems, the average cation-cation interaction strength increased significantly in this double mixed material, indicating a restriction of the space accessible to the cations or their partial immobilization upon Br- incorporation. Our results shed light on the heterogeneities in cation composition of mixed halide perovskites, helping to exploit their full optoelectronic potential.

How antisolvent miscibility affects perovskite film wrinkling and photovoltaic properties.

Charge carriers' density, their lifetime, mobility, and the existence of trap states are strongly affected by the microscopic morphologies of perovskite films, and have a direct influence on the photovoltaic performance. Here, we report on micro-wrinkled perovskite layers to enhance photocarrier transport performances. By utilizing temperature-dependent miscibility of dimethyl sulfoxide with diethyl ether, the geometry of the microscopic wrinkles of the perovskite films are controlled. Wrinkling is pronounced as temperature of diethyl ether (TDE) decreases due to the compressive stress relaxation of the thin rigid film-capped viscoelastic layer. Time-correlated single-photon counting reveals longer carrier lifetime at the hill sites than at the valley sites. The wrinkled morphology formed at TDE = 5 °C shows higher power conversion efficiency (PCE) and better stability than the flat one formed at TDE = 30 °C. Interfacial and additive engineering improve further PCE to 23.02%. This study provides important insight into correlation between lattice strain and carrier properties in perovskite photovoltaics.

Role of Torsional Flexibility in the Film Formation Process in Two π-Conjugated Model Oligomers.

The performance of solution-processed organic semiconductor devices is heavily influenced by the morphology of the active layer. Film formation is a complex process, with the final morphology being the result of the interplay between processing parameters and molecular properties, which is only poorly understood. Here, we investigate the influence of molecular stiffness by using two model oligomers, TT and CT, which differ only in the rotational flexibility of their central building block. We monitor absorption and emission simultaneously in situ during spin coating. We find that film formation takes place in four similar stages for both compounds. However, the time scales are remarkably different during the third stage, where electronically interacting aggregates are created. While this process is fast for the stiff CT, it takes minutes for the flexible TT. By comparing with previously determined aggregation properties in solution, we conclude that even though aggregate formation concurs with a planarization process, a certain amount of backbone flexibility is beneficial for establishing ordered structures during film formation. Here, the elongated time window in the case of the flexible compound can further allow for better processing control.

High Versatility and Stability of Mechanochemically Synthesized Halide Perovskite Powders for Optoelectronic Devices.

We show that mechanochemically synthesized halide perovskite powders from a ball milling approach can be employed to fabricate a variety of lead halide perovskites with exceptional intrinsic stability. Our MAPbI3 powder exhibits higher thermal stability than conventionally processed thin films, without degradation after more than two and a half years of storage and only negligible degradation after heat treatment at 220 °C for 14 h. We further show facile recovery strategies of nonphase-pure powders by simple remilling or mild heat treatment. Moreover, we demonstrate the mechanochemical synthesis of phase-pure mixed perovskite powders, such as (Cs0.05FA0.95PbI3)0.85(MAPbBr3)0.15, from either the individual metal and organic halides or from readily prepared ternary perovskites, regardless of the precursor phase purity. Adding potassium iodide (KI) to the milling process successfully passivated the powders. We also succeeded in preparing a precursor solution on the basis of the powders and obtained uniform thin films for integration into efficient perovskite solar cells from spin-coating this solution. We find the KI passivation remains in the devices, leading to improved performance and significantly reduced hysteresis. Our work thus demonstrates the potential of mechanochemically synthesized halide perovskite powders for long-time storage and upscaling, further paving the way toward commercialization of perovskite-based optoelectronic devices.

Temperature Induced Order-Disorder Transition in Solutions of Conjugated Polymers Probed by Optical Spectroscopy.

The aggregation of π-conjugated materials significantly impacts the photophysics and performance of optoelectronic devices. Nevertheless, little is known about the laws governing aggregate formation of π-conjugated materials from solution. In this Perspective, we compare, discuss, and summarize how aggregates form for three different types of compounds, that is, homopolymers, donor-acceptor type polymers, and low molecular weight compounds. To this end, we employ temperature-dependent optical spectroscopy, which is a simple yet powerful tool to investigate aggregate formation. We show how optical spectra can be analyzed to identify distinct conformational states. We find aggregate formation to proceed the same in all these compounds by a coil-to-globule-like first-order phase transition. Notably, the chain expands before it collapses into a highly ordered dense state. The role of side chains and the impact of changes in environmental polarization are addressed.

Compact Layers of Hybrid Halide Perovskites Fabricated via the Aerosol Deposition Process-Uncoupling Material Synthesis and Layer Formation.

We present the successful fabrication of CH3NH3PbI3 perovskite layers by the aerosol deposition method (ADM). The layers show high structural purity and compactness, thus making them suitable for application in perovskite-based optoelectronic devices. By using the aerosol deposition method we are able to decouple material synthesis from layer processing. Our results therefore allow for enhanced and easy control over the fabrication of perovskite-based devices, further paving the way for their commercialization.

Effect of Thermal and Structural Disorder on the Electronic Structure of Hybrid Perovskite Semiconductor CH3NH3PbI3.

In this Letter, we investigate the temperature dependence of the optical properties of methylammonium lead iodide (MAPbI3 = CH3NH3PbI3) from room temperature to 6 K. In both the tetragonal (T > 163 K) and the orthorhombic (T < 163 K) phases of MAPbI3, the band gap (from both absorption and photoluminescence (PL) measurements) decreases with decrease in temperature, in contrast to what is normally seen for many inorganic semiconductors, such as Si, GaAs, GaN, etc. We show that in the perovskites reported here, the temperature coefficient of thermal expansion is large and accounts for the positive temperature coefficient of the band gap. A detailed analysis of the exciton line width allows us to distinguish between static and dynamic disorder. The low-energy tail of the exciton absorption is reminiscent of Urbach absorption. The Urbach energy is a measure of the disorder, which is modeled using thermal and static disorder for both the phases separately. The static disorder component, manifested in the exciton line width at low temperature, is small. Above 60 K, thermal disorder increases the line width. Both these features are a measure of the high crystal quality and low disorder of the perovskite films even though they are produced from solution.

Ultrafast Energy Transfer between Disordered and Highly Planarized Chains of Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV).

Upon cooling a solution of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), a phase transition occurs, leading to the formation of aggregates. We have studied the dynamics of singlet excitons in MEH-PPV solution below the critical temperature of the phase transition using steady-state photoluminescence measurements and pump-probe fs-spectroscopy at different temperatures. Spectral analysis indicates the coexistence of disordered chromophores with highly planarized chromophores. The high planarity is evidenced by a remarkably high 0-0/0-1 peak ratio in the spectra. By spectrally separating the contributions of either type of chromophore to the pump-probe signal we find that energy transfer takes place within less than 1 ps from disordered, unaggregated chain segments to highly planarized, aggregated chain segments. The short time scale of the energy transfer indicates intimate intermixing of the planarized and disordered polymeric chromophores.